Program.cs

using System;
using System.IO;
using System.IO.Compression;
using System.Net.Http;
using System.Threading.Tasks;

namespace ConsoleApp2
{
    class Program
    {
        static async Task Main(string[] args)
        {
            using (var httpClient = new HttpClient())
            using (var packageStream = new MemoryStream())
            {
                using (var response = await httpClient.GetAsync("https://api.nuget.org/v3-flatcontainer/fantas.monogame/1.0.5/fantas.monogame.1.0.5.nupkg"))
                using (var stream = await response.Content.ReadAsStreamAsync())
                {
                    await stream.CopyToAsync(packageStream);
                }

                using (var zip = new ZipArchive(packageStream, ZipArchiveMode.Read, leaveOpen: true))
                {
                    foreach (var entry in zip.Entries)
                    {
                        Console.WriteLine(entry.FullName);
                    }
                }
            }
        }
    }
}

There's an extra space on line 13.

This serious transgression has been resolved. Thanks for bringing it to my attention 😂